This invention relates to digitizer systems and, more particularly, in an electromagnetic digitizer tablet having a grid of receptor wires disposed below a working surface over which a cursor emanating a magnetic field detected by the receptor wires is moved, to the improvement to reduce electrostatically-caused jitters in data from the receptor wires comprising a grounded electrically conductive shield disposed between the grid of receptor wires and the working surface, the grounded electrically conductive shield being of a material and thickness to pass the magnetic field without substantial attenuation while conducting any electrostatic energy forming thereon to ground. It also relates to digitizers as employed as part of the input/display device of a pen-driven computing system.
In a digitizer system such as that indicated as 10 in FIG. 1, a cursor 12 connected to a tablet 14 by a connecting cable 16 is moved over a working surface 18 of the tablet 14 in order to input positional data to a computer (not shown) connected to the tablet 14. As shown in the cutaway drawing of FIG. 2, the tablet 14 typically comprises a plastic upper surface 20 carrying a grid of electrical conductors 22 which interact with a coil 24 within the cursor 12 to provide the positional information required. In such systems, it is quite typical to provide a metallic shield 26 of aluminum, or the like, over the grid of conductors 22. The shield 26 acts to magnetically shunt and concentrate electromagnetic radiation from the coil 24 thereby eliminating problems of stray fields interacting with undesired wires at the peripheral edges of the grid of conductors 22 in particular. Where the tablet 14 is mounted in a case 27 as depicted in FIG. 3, the metallic shield 26' may be attached to the bottom of the case 27 as with adhesive or the like.
While earlier digitizing systems 10, such as those of FIGS. 1, 2, and 3 employed printed circuit boards with the grid of conductors 22 formed thereon either as conductors according to printed circuit techniques or as actual wires physically connected to the circuitry of the printed circuit board, more recently the grid of conductors 22 has been formed on a sheet of mylar, or the like, employing silk-screening techniques with conductive inks. The mylar substrate containing the printed grid of conductors 22 is then wrapped over and around the edges of a supporting piece of insulating material such as a printed circuit board, which may or may not contain additional components and logic associated with the digitizer system.
When a digitizer tablet constructed according to such techniques is placed close adjacent a computer terminal, electrostatic radiation from the terminal may cause data jitters in the electromagnetic digitizer. In such construction, the use of a metal shield 26, as in FIG. 2, is not practical and would not solve the particular problem as it is functionally positioned to shunt a portion of electromagnetic waves and not to pass electromagnetic waves and shunt electrostatic radiation as is necessary to solve the problem addressed by the present invention. This is particularly true in larger-sized digitizer tablets employed with larger drawings, and the like. A large metal shield associated with the tablet is impractical and unsuitable for solving the problem on the one hand and, even if it could solve the electrostatic radiation problem, would be cost-prohibitive on the other hand. Electrostatic interference is also a problem with so-called pen-driven computing systems where a digitizing tablet is associated with a back-lit liquid crystal display panel wherein both the liquid crystal display panel and the backlighting panel behind it emit electrostatic energy.
Wherefore, it is an object of the present invention to provide a method and apparatus for shielding conductive ink flexible digitizer tablets from the effects of electrostatic radiation while not interfering with required electromagnetic fields.
It is another object of the present invention to provide a method and apparatus for shielding conductive ink flexible digitizer tablets associated with back-lit liquid crystal display panels in pen-driven computing systems from the electrostatic radiations of the liquid crystal display panel and the backlighting panel behind it.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.